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Abstract:

A resist pattern is formed by coating a first positive resist composition
comprising a polymer comprising 20-100 mol % of aromatic group-containing
recurring units and adapted to turn alkali soluble under the action of an
acid onto a substrate to form a first resist film, coating a second
positive resist composition comprising a C3-C8 alkyl alcohol
solvent which does not dissolve the first resist film on the first resist
film to form a second resist film, exposing, baking, and developing the
first and second resist films simultaneously with a developer.

Claims:

1. A pattern forming process comprising coating a first positive resist
composition onto a substrate to form a first resist film, said first
resist composition comprising a first polymer comprising 20 mol % to 100
mol % of aromatic group-containing recurring units and adapted to turn
alkali soluble under the action of an acid and a first solvent, coating a
second positive resist composition on the first resist film to form a
second resist film, said second resist composition comprising a second
polymer and a second solvent of C3-C8 alkyl alcohol which does
not dissolve the first resist film, exposing to high-energy radiation
having a wavelength, post-exposure baking, and developing the first and
second resist films simultaneously with a developer to form a resist
pattern.

2. The process of claim 1 wherein the first resist film has an extinction
coefficient (k value) in the range of 0.1 to 1.1 at the exposure
wavelength.

3. The process of claim 1 wherein the radiation for exposure is ArF
excimer laser radiation having a wavelength of 193 nm.

4. The process of claim 1 wherein the aromatic group in the first resist
composition is benzene ring.

5. The process of claim 1 wherein the first solvent in the first resist
composition is selected from the group consisting of ketones, ether or
ester-containing alcohols, ethers, esters, and lactones, and mixtures
comprising at least two of the foregoing.

8. The process of claim 1 wherein the second polymer in the second resist
composition has a 2,2,2-trifluoro-1-hydroxyethyl group.

9. The process of claim 8 wherein the second polymer comprises recurring
units having a 2,2,2-trifluoro-1-hydroxyethyl group, represented by the
general formula (1): ##STR00089## wherein R1 is hydrogen or
methyl, m is 1 or 2, R2 is a straight, branched or cyclic
C1-C10 alkylene group, which may contain an ester, ether,
hydroxyl group or fluorine, in the case of m=1, or R2 is the
foregoing alkylene group with one hydrogen atom eliminated in the case of
m=2, or R2 and R3 may bond together to form a ring, and R3
is hydrogen, fluorine, methyl, trifluoromethyl or difluoromethyl.

10. The process of claim 8 wherein the second polymer is a copolymer
comprising recurring units (a) having a 2,2,2-trifluoro-1-hydroxyethyl
group and recurring units (b1) having an acid labile group, represented
by the general formula (2): ##STR00090## wherein R1 to R2 and
m are as defined above, R4 is hydrogen or methyl, R5 is an acid
labile group, a and b1 are numbers in the range: 0<a<1.0,
0<b1<b1<1.0, and 0<a+b1.ltoreq.1.0.

11. The process of claim 1 wherein the first polymer in the first resist
composition comprises recurring units (b2), (c1), and (c2) as represented
by the general formula (3): ##STR00091## wherein R6, R9, and
R13 each are hydrogen or methyl, X1, X2, and X3 each
are a single bond or --C(═O)--O--, X2 may also be
--C(═O)--NH--, R7, R10, and R14 each are a single bond
or a straight or branched C1-C6 alkylene group, which may
contain an ether group, ester group or lactone ring, R8, R11,
and R15 each are an acid labile group, R12 is hydrogen,
fluorine or a straight, branched or cyclic C1-C6 alkyl group, p
and s are 1 or 2, q is an integer of 0 to 4, r1 and r2 are an
integer of 0 to 2, b2, c1, and c2 are numbers in the range:
0.ltoreq.b2<1.0, 0.ltoreq.c1<1.0, 0.ltoreq.c2<1.0,
0.2<b2+c1+c2.ltoreq.1.0.

12. The process of claim 11 wherein the first polymer further comprise
recurring units (d) having an adhesive group selected from the group
consisting of lactone ring, carbonate, thiocarbonate, carbonyl, cyclic
acetal, ether, ester, sulfonic acid ester, cyano, amide groups, and
--O--C(═O)--Y-- wherein Y is sulfur or NH.

13. The process of claim 1, further comprising, after the developing
step, the step of processing the substrate by dry etching through the
resist pattern.

14. The process of claim 1, further comprising, after the developing
step, the step of processing the substrate by ion implantation through
the resist pattern.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This non-provisional application claims priority under 35 U.S.C.
§119(a) on Patent Application No. 2010-246185 filed in Japan on Nov.
2, 2010, the entire contents of which are hereby incorporated by
reference.

TECHNICAL FIELD

[0002] This invention relates to a pattern forming process comprising the
steps of forming a first photoresist film having high absorption at
exposure wavelength on a substrate, coating thereon a second resist
material comprising an alkyl alcohol solvent which does not dissolve the
first photoresist film to form a second photoresist film, exposure, and
development, thereby simultaneously forming first and second resist
patterns.

BACKGROUND ART

[0003] One recent approach for the manufacture of CMOS devices is to
effect ion implantation on a substrate through a KrF resist film as a
mask in order to form p- and n-wells in the substrate. As the resist
pattern is reduced in size, a replacement by a ArF resist film is in
progress, and the ArF immersion lithography is proposed for further
miniaturization. In order to carry out ion implantation, the substrate
surface must be exposed through space portions in the resist film
pattern. If a bottom antireflective coating (BARC) layer is present
beneath the resist film, ions can be trapped by the BARC layer. However,
if the photoresist film is patterned in the absence of the BARC layer,
then standing waves generate due to substrate reflection, whereby the
resist pattern after development has substantially ridged sidewalls. For
the purpose of smoothening such ridges due to standing waves, it is
believed effective to enhance acid diffusion by using a photoacid
generator (PAG) capable of generating a low molecular weight acid which
is prone to diffuse and performing PEB at higher temperature. As long as
the size at which the resist film subject to ion implantation is resolved
by the KrF lithography is in the range of 200 to 300 nm, it is not
recognized that resolution is degraded by enhancement of acid diffusion.
However, when the size at which the resist film subject to ion
implantation is resolved by the ArF lithography is reduced to less than
200 nm, undesirably the enhancement of acid diffusion can cause
degradation of resolution and increase the proximity bias.

[0004] The most traditional means for preventing generation of standing
waves is a dyed resist material for forming a photoresist film which is
absorptive in itself. The study on that means started from the novolac
resist materials for i and g-line exposure. As the absorptive component
which can be used in the ArF lithography, a study was made on the
introduction of benzene ring into a base polymer and the use of an
additive having benzene ring. However, it is impossible for the
absorptive component to completely prevent standing waves. If the
component is made more absorptive, standing waves are reduced, but the
cross-sectional profile of a resist pattern can be tapered into a
trapezoidal shape.

[0005] It is also studied to form an antireflective coating (TARC) on top
of a resist film. TARC is effective for reducing standing waves, but not
effective for preventing halation due to irregularities on the substrate.
Ideally, the refractive index of TARC is equal to the square root of the
refractive index of a photoresist film. Since a methacrylate polymer used
in the ArF resist film has a relatively low refractive index of 1.7 at
the wavelength 193 nm, there is available few materials having a low
refractive index equal to the square root of that refractive index, 1.30.

[0006] The study was then made on developer-soluble bottom antireflective
coating (DBARC, see Non-Patent Documents 1 and 2). The early study was
focused on the BARC which is soluble non-isotropically in developer. This
approach encountered difficulty of size control in that undercuts were
formed beneath the resist pattern upon excessive progress of dissolution,
whereas scum was left in space portions upon shortage of dissolution.
Studied next was photosensitive BARC. In order for a material to function
as BARC, the material should have an antireflective effect, and when a
photoresist solution is coated thereon to form a photoresist film, the
material should not dissolve in the photoresist solution and be devoid of
intermixing with the photoresist film. The dissolution in photoresist
solution and the intermixing can be prevented by inducing crosslinking
during bake after coating of a BARC solution. If BARC is provided with a
positive photosensitive function, the exposed region must dissolve in
developer like the photoresist film. The crosslinked BARC film has the
problem that dissolution in developer is difficult even after
deprotection of acid labile groups, leaving scum in the space region.

[0007] In the case of the prior art crosslinking BARC, after resist
development to form a resist pattern, the substrate is processed by dry
etching through the resist pattern serving as a mask. There is a problem
that the resist pattern is reduced in film thickness when the BARC film
is opened. Thus a BARC having a high etching rate is required. Since the
BARC film is almost unexpectable to have etch resistance, an improvement
in etch resistance to enable substrate processing is assigned to the
resist film. As the resist film is made thinner, the etching resistance
becomes lower. An attempt to provide BARC with etching resistance to
enable substrate processing encounters a dilemma that the resist pattern
is more damaged when the BARC film is opened.

[0010] The photoresist film used in the ion implantation process must
allow the substrate surface in the exposed portion thereof to be opened
after development in order to implant ions into an opened region. In this
case, a silicon substrate is commonly used as the substrate, and so
reflection from the substrate is substantial. TARC is effective for
suppressing standing waves. However, since an optimum low refractive
material capable of completely suppressing standing waves is not
available, TARC allows standing waves to generate and lacks the effect of
suppressing diffuse reflection or halation in the presence of
irregularities on the substrate. In the case of a resist film containing
an absorptive component, a highly absorptive component is more effective
for suppressing substrate reflection, but leads to a tapered profile,
whereas a component which is less absorptive so as not to cause a tapered
profile is less effective for suppressing substrate reflection and allows
for formation of ridges due to standing waves. BARC has a very high
anti-reflection effect because reflection is suppressed in two ways,
i.e., by light absorption of an absorber and offsetting between incident
light and reflected light by a choice of optimum film thickness. However,
the BARC surface appears after development and inhibits ion implantation
into the substrate interior. The photosensitive BARC has the problem that
the solubility of an exposed portion of BARC in alkaline developer is
reduced due to crosslinking during post-spin-coating bake for preventing
mixing with the resist film, leaving scum on the substrate surface in the
exposed portion of BARC. It would be desirable to have a photosensitive
BARC which leaves no scum on the substrate surface.

[0011] An object of the invention is to provide a pattern forming process
using a first photoresist film that is fully effective as a
photosensitive antireflective film which leaves no scum on the substrate
surface and is improved in size control, and a second photoresist film to
be deposited thereon.

[0012] The invention provides a pattern forming process comprising
[0013] coating a first positive resist composition onto a substrate to
form a first resist film, said first resist composition comprising a
first polymer comprising 20 mol % to 100 mol % of aromatic
group-containing recurring units and adapted to turn alkali soluble under
the action of an acid and a first solvent, [0014] coating a second
positive resist composition on the first resist film to form a second
resist film, said second resist composition comprising a second polymer
and a second solvent of C3-C8 alkyl alcohol which does not
dissolve the first resist film, [0015] exposing to high-energy radiation
having a wavelength, post-exposure baking, and [0016] developing the
first and second resist films simultaneously with a developer to form a
resist pattern.

[0017] In a preferred embodiment, the first resist film has an extinction
coefficient (k value) in the range of 0.1 to 1.1 at the exposure
wavelength. Typically the radiation for exposure is ArF excimer laser
radiation having a wavelength of 193 nm.

[0018] In a preferred embodiment, the aromatic group in the first resist
composition is benzene ring.

[0021] In a preferred embodiment, the second polymer in the second resist
composition has a 2,2,2-trifluoro-1-hydroxyethyl group. More preferably,
the second polymer comprises recurring units having a
2,2,2-trifluoro-1-hydroxyethyl group, represented by the general formula
(1):

##STR00001##

wherein R1 is hydrogen or methyl, m is 1 or 2, R2 is a
straight, branched or cyclic C1-C10 alkylene group, which may
contain an ester, ether, hydroxyl group or fluorine, in the case of m=1,
or R2 is the foregoing alkylene group with one hydrogen atom
eliminated in the case of m=2, or R2 and R3 may bond together
to form a ring, and R3 is hydrogen, fluorine, methyl,
trifluoromethyl or difluoromethyl.

[0022] Also preferably, the second polymer is a copolymer comprising
recurring units (a) having a 2,2,2-trifluoro-1-hydroxyethyl group and
recurring units (b1) having an acid labile group, represented by the
general formula (2):

##STR00002##

wherein R1 to R3 and m are as defined above, R4 is
hydrogen or methyl, R5 is an acid labile group, a and b1 are numbers
in the range: 0<a<1.0, 0<b1<1.0, and 0<a+b1<1.0.

[0023] In a preferred embodiment, the first polymer in the first resist
composition comprises recurring units (b2), (c1), and (c2) as represented
by the general formula (3):

##STR00003##

wherein R6, R9, and R13 each are hydrogen or methyl,
X1, X2, and X3 each are a single bond or --C(═O)--O--,
X2 may also be --C(═O)--NH--, R7, R10, and R14
each are a single bond or a straight or branched C1-C6 alkylene
group, which may contain an ether group, ester group or lactone ring,
R8, R11, and R15 each are an acid labile group, R12
is hydrogen, fluorine or a straight, branched or cyclic C1-C6
alkyl group, p and s are 1 or 2, q is an integer of 0 to 4, r1 and
r2 are an integer of 0 to 2, b2, c1, and c2 are numbers in the
range: 0≦b2<1.0, 0≦c1<1.0, 0≦c2<1.0,
0<b2+c1+c2≦1.0.

[0024] More preferably, the first polymer further comprise recurring units
(d) having an adhesive group selected from the group consisting of
lactone ring, carbonate, thiocarbonate, carbonyl, cyclic acetal, ether,
ester, sulfonic acid ester, cyano, amide groups, and --O--C(═O)--Y--
wherein Y is sulfur or NH.

[0025] In a preferred embodiment, the process further comprises, after the
developing step, the step of processing the substrate by dry etching
through the resist pattern, or the step of processing the substrate by
ion implantation through the resist pattern.

[0026] When the pattern forming process defined above is applied,
formation of ridges due to standing waves in the cross section of the
photoresist pattern after development is prevented because reflection
from the substrate is suppressed. The substrate surface in the exposed
portion after development can be opened at a high degree of size control
without scumming.

Advantageous Effects of the Invention

[0027] According to the invention, a first positive resist composition
comprising a polymer containing aromatic groups in 20 mol % to 100 mol %
of recurring units and adapted to turn alkali soluble under the action of
an acid is coated onto a substrate to form a first resist film. A second
positive resist composition comprising a solvent of C3-C8 alkyl
alcohol which does not dissolve the first resist film is coated on the
first resist film to form a second resist film. Exposure is performed
while the influence of standing waves or halation by reflection from the
substrate during exposure is eliminated. This is followed by PEB and
simultaneous development of the first and second resist films with a
developer to form a resist pattern. The substrate surface can be opened
after development.

[0028] Because of the high content of aromatic groups, the first resist
film has higher etch resistance than the second resist film. The second
resist film combined with the first resist film offers high resistance to
etching of the substrate using the resist pattern after development as a
mask or high resistance to ion implantation, as compared with the second
resist film used alone.

BRIEF DESCRIPTION OF DRAWINGS

[0029]FIG. 1 is a cross-sectional view of a pattern forming process
according one embodiment of the invention. FIG. 1A shows a processable
layer and a first resist film deposited on a substrate, FIG. 1B shows a
second resist film deposited thereon, FIG. 1C shows exposure, FIG. 1D
shows development, and FIG. 1E shows the processable layer etched using
the resist pattern as a mask.

[0030] FIG. 2 is a cross-sectional view of a pattern forming process
according another embodiment of the invention. FIG. 2A shows a first
resist film deposited on a substrate, FIG. 2B shows a second resist film
deposited thereon, FIG. 2C shows exposure, FIG. 2D shows development, and
FIG. 2E shows ion implantation into the substrate using the resist
pattern as a mask.

[0031] FIG. 3 is a diagram showing reflectivity from beneath a first
resist film having an n value of 1.3 when the k value and thickness of
the resist film are varied.

[0032] FIG. 4 is a diagram showing reflectivity from beneath a first
resist film having an n value of 1.4 when the k value and thickness of
the resist film are varied.

[0033] FIG. 5 is a diagram showing reflectivity from beneath a first
resist film having an n value of 1.5 when the k value and thickness of
the resist film are varied.

[0034] FIG. 6 is a diagram showing reflectivity from beneath a first
resist film having an n value of 1.6 when the k value and thickness of
the resist film are varied.

[0035] FIG. 7 is a diagram showing reflectivity from beneath a first
resist film having an n value of 1.7 when the k value and thickness of
the resist film are varied.

[0036] FIG. 8 is a diagram showing reflectivity from beneath a first
resist film having an n value of 1.8 when the k value and thickness of
the resist film are varied.

[0037] FIG. 9 is a diagram showing reflectivity from beneath a first
resist film having an n value of 1.9 when the k value and thickness of
the resist film are varied.

[0038] FIG. 10 is a diagram showing reflectivity from beneath a first
resist film having an n value of 2.0 when the k value and thickness of
the resist film are varied.

DESCRIPTION OF THE EMBODIMENTS

[0039] The singular forms "a," "an" and "the" include plural referents
unless the context clearly dictates otherwise. "Optional" or "optionally"
means that the subsequently described event or circumstances may or may
not occur, and that description includes instances where the event or
circumstance occurs and instances where it does not. As used herein, the
terminology "(Cx-Cy)", as applied to a particular unit, such
as, for example, a chemical compound or a chemical substituent group,
means having a carbon atom content of from "x" carbon atoms to "y" carbon
atoms per such unit. As used herein, the term "film" is used
interchangeably with "coating" or "layer." The term "processable layer"
refers to a layer that can be processed typically by etching or ion
implantation.

[0040] The abbreviations and acronyms have the following meaning.

[0041] ARC: antireflective coating

[0042] Mw: weight average molecular weight

[0043] Mn: number average molecular weight

[0044] Mw/Mn: molecular weight distribution or dispersity

[0045] GPC: gel permeation chromatography

[0046] PEB: post-exposure baking

[0047] The invention pertains to the photolithography of forming a pattern
on a highly reflective substrate, typically silicon substrate so that the
substrate may be ready for ion implantation. The inventors made research
on a patterning process capable of preventing reflection from the
substrate and opening the exposed portion without scumming in the opening
as found on use of DBARC, and a photoresist material used in the process.

[0048] The inventors have found that a pattern can be formed
simultaneously from first and second resists by using a positive resist
composition having strong absorption at the exposure wavelength as the
first positive resist material, coating thereon a second positive resist
composition comprising a solvent of C3-C8 alkyl alcohol which
does not dissolve the first resist film, and effecting exposure and
development, and that the substrate surface in the exposed portion can be
opened while suppressing the generation of ridges on sidewalls of the
resist pattern due to reflection from the substrate and generation of
standing waves and without leaving scum.

[0049] Specifically, since the first resist film need not be crosslinked
as is DBARC, no scum is generated in the exposed portion. While the first
resist film has absorption equivalent to BARC, it can be improved in
contrast due to the lack of crosslinking, which makes it possible to
prevent a tapered profile and scumming.

[0050] While both the first and second positive resist compositions are
chemically amplified positive resist compositions, at least first
positive resist film should be insoluble in the solvent in the second
positive resist composition. If a base polymer in the first positive
resist composition has lactone as a predominant adhesive group, this
polymer is difficulty soluble in alcohol solvents. On the other hand, a
base polymer in the second positive resist composition should essentially
contain a weakly acidic hydroxyl group in order that the polymer be
soluble in alcohol solvents. The weakly acidic hydroxyl groups are
α-trifluoromethyl hydroxy groups as typified by
2,2,2-trifluoro-1-hydroxyethyl and phenol groups. Although the phenol
group cannot be used for the base polymer in the second resist
composition because of strong absorption of benzene ring at the
wavelength 193 nm, the naphthol group is applicable because the
absorption peak wavelength is shifted to the longer wavelength side.

[0051] The first resist composition should meet not only the function of
photoresist material, but also the function of ARC. In the case of ArF
excimer laser having an exposure wavelength of 193 nm, the absorbing
group is most preferably benzene ring. Benzene ring may be contained as
an acid labile group, or an adhesive group, or a styrene directly bonded
to a polymeriable group. The amount of benzene ring-containing recurring
units is 20 to 100 mol % provided that the entire polymer units total to
100 mol %.

[0052] Benzene ring may be introduced in different ways, for example, as
an acid labile group, or an adhesive group such as hydroxystyrene having
a phenol group, or as non-polar recurring units like styrene and indene,
or as a rigidity-enhancing adhesive group like coumarin or chromone.
Moreover, benzene ring may be introduced as recurring units having an
acid labile group-substituted phenolic hydroxyl group or carboxyl group,
specifically recurring units (c1) and (c2) in the following general
formula (3).

##STR00004##

Herein R6, R9, and R13 each are hydrogen or methyl,
X1, X2, and X3 each are a single bond or --C(═O)--O--,
X2 may also be --C(═O)--NH--, R7, R10, and R14
each are a single bond or a straight or Branched C1-C6alkylene
group, which may contain an ether group, ester group or lactone ring,
R8, R11, and R15 each are an acid labile group, R12
is hydrogen, fluorine or a straight, branched or cyclic C1-C6
alkyl group, p and s are 1 or 2, q is an integer of 0 to 4, r1 and
r2 are an integer of 0 to 2, b2, c1, and c2 are numbers in the
range: 0≦b2<1.0, 0≦c1<1.0, 0≦c2<1.0,
0<b2+c1+c2≦1.0.

[0053] In order that the first resist composition function as positive
resist material, the first polymer should comprise recurring units of at
least one type selected from recurring units having an acid labile
group-substituted carboxyl group or phenolic hydroxyl group (b2), (c1)
and (c2) in formula (3).

[0054] The monomer from which recurring units (b2) are derived is a
methacrylate or acrylate monomer as shown below.

##STR00005##

Herein R6 to R8 and X1 are as defined above.

[0055] Examples of the monomer from which recurring units (b2) are derived
are shown below.

##STR00006## ##STR00007##

[0056] Examples of the monomers from which recurring units (c1) and (c2)
are derived are shown below.

##STR00008## ##STR00009## ##STR00010## ##STR00011## ##STR00012##

[0057] In addition to recurring units (b2), (c1) and (c2) as represented
by formula (3), the first polymer used in the first resist composition
may have further copolymerized therein recurring units (d) having an
adhesive group selected from among lactone ring, carbonate,
thiocarbonate, carbonyl, cyclic acetal, ether, ester, sulfonic acid
ester, cyano, amide groups, and --O--C(═O)--Y-- wherein Y is sulfur
or NH. Then the polymer becomes substantially insoluble in the solvent of
C3-C8 alkyl alcohol to form a resist solution for the second
resist film, specifically n-propanol, isopropyl alcohol, 1-butyl alcohol,
2-butyl alcohol, isobutyl alcohol, tert-butyl alcohol, 1-pentanol,
2-pentanol, 3-pentanol, tert-amyl alcohol, neopentyl alcohol,
2-methyl-1-butanol, 3-methyl-1-butanol, 3-methyl-3-pentanol,
cyclopentanol, 1-hexanol, 2-hexanol, 3-hexanol, 2,3-dimethyl-2-butanol,
3,3-dimethyl-1-butanol, 3,3-dimethyl-2-butanol, 2-diethyl-1-butanol,
2-methyl-1-pentanol, 2-methyl-1-pentanol, 2-methyl-3-pentanol,
3-methyl-1-pentanol, 3-methyl-2-pentanol, 3-methyl-3-pentanol,
4-methyl-1-pentanol, 4-methyl-2-pentanol, 4-methyl-3-pentanol,
1-heptanol, cyclohexanol, and octanol. When the second resist composition
is dispensed on the first resist film, the first resist film is not
dissolved therein or intermixed with the second resist film.

[0059] The first polymer in the first resist composition may have further
copolymerized therein recurring units (e) having a phenolic hydroxyl
group, which are exemplified below.

##STR00025## ##STR00026## ##STR00027## ##STR00028##

[0060] It is acceptable that in the monomer stage, the phenolic hydroxyl
group is protected with an acid labile group or acyl group, and after
polymerization, it is deprotected with an acid or alkaline aqueous
solution to restore a hydroxyl group.

[0062] The second polymer, which is combined with the C3-C8
alkyl alcohol solvent to form the second positive resist composition,
should preferably comprise recurring units (a) having a
2,2,2-trifluoro-1-hydroxyethyl group, specifically represented by the
general formula (1).

##STR00029##

Herein R1 is hydrogen or methyl, and m is 1 or 2. R2 is a
straight, branched or cyclic C1-C10 alkylene group, which may
contain an ester, ether, hydroxyl group or fluorine, in the case of m=1.
R2 is the foregoing alkylene group with one hydrogen atom eliminated
in the case of m=2. Alternatively, R2 and R3 bond together to
form a C3-C10 aliphatic ring with the carbon atom to which they
are attached. R3 is hydrogen, fluorine, methyl, trifluoromethyl or
difluoromethyl.

[0063] Examples of the monomer from which recurring units (a) are derived
are given below.

[0064] Preferably the second polymer used as a base resin in the second
positive resist composition comprises recurring units (a) having a
2,2,2-trifluoro-1-hydroxyethyl group and recurring units (b) having an
acid labile group as represented by the general formula (2).

##STR00039##

Herein R1 to R3 and m are as defined above, R4 is hydrogen
or methyl, R5 is an acid labile group, a and b1 are numbers in the
range: 0<a<1.0, 0<b1<1.0, and 0<a+b1≦1.0.

[0065] The monomers from which recurring units (b1) are derived are
represented by the following formula.

##STR00040##

Herein R4 and R5 are as defined above.

[0066] In the second polymer, a monomer corresponding to recurring units
(e) having a phenolic hydroxyl group, as described above, may be
copolymerized.

[0067] Further, the second polymer may have copolymerized therein
recurring units (g) having a hydroxyl group, as described below, for
enhancing the solubility in the alcohol solvent. It is acceptable that
the recurring units having a hydroxyl group be introduced into the first
polymer. Since the introduction of recurring units having a hydroxyl
group enhances the effect of controlling acid diffusion, an improvement
in the lithographic performance by controlled acid diffusion is expected
not only for the second resist composition, but also for, the first
resist composition.

[0068] Examples of the recurring units (g) having a hydroxyl group are
shown below.

##STR00041## ##STR00042## ##STR00043## ##STR00044##

[0069] Further, the second polymer may have copolymerized therein
recurring units (h) having a sulfonamide group, as illustrated below.
This also enhances the solubility of the polymer in the alcohol solvent.

##STR00045## ##STR00046##

[0070] In addition to recurring units (a) and acid labile group-containing
recurring units (b1) as represented by formula (2), the second polymer
used in the second resist composition may have further copolymerized
therein recurring units (d) having an adhesive group selected from among
lactone ring, carbonate, thiocarbonate, carbonyl, cyclic acetal, ether,
ester, sulfonic acid ester, cyano, amide groups, and --O--C(═O)--Y--
wherein Y is sulfur or NH, as described above.

[0071] The acid labile group represented by R5, R8, R11 and
R15 in formulae (2) and (3) may be selected from a variety of such
groups. The acid labile groups may be the same or different and
preferably include groups of the following formulae (A-1) and (A-2),
tertiary alkyl groups of the following formula (A-3), and oxoalkyl groups
of 4 to 20 carbon atoms.

##STR00047##

[0072] In formula (A-1), e is a tertiary alkyl group of 4 to 20 carbon
atoms, preferably 4 to 15 carbon atoms, a trialkylsilyl group in which
each alkyl moiety has 1 to 6 carbon atoms, an oxoalkyl group of 4 to 20
carbon atoms, or a group of formula (A-3). Exemplary tertiary alkyl
groups are tert-butyl, tert-amyl, 1,1-diethylpropyl, 1-ethylcyclopentyl,
1-butylcyclopentyl, 1-ethylcyclohexyl, 1-butylcyclohexyl,
1-ethyl-2-cyclopentenyl, 1-ethyl-2-cyclohexenyl, and
2-methyl-2-adamantyl. Exemplary trialkylsilyl groups are trimethylsilyl,
triethylsilyl, and dimethyl-tert-butylsilyl. Exemplary oxoalkyl groups
are 3-oxocyclohexyl, 4-methyl-2-oxooxan-4-yl, and
5-methyl-2-oxooxolan-5-yl. Letter a1 is an integer of 0 to 6.

[0073] In formula (A-2), R31 and R32 are hydrogen or straight,
branched or cyclic alkyl groups of 1 to 18 carbon atoms, preferably 1 to
10 carbon atoms. Exemplary alkyl groups include methyl, ethyl, propyl,
isopropyl, n-butyl, sec-butyl, tert-butyl, cyclopentyl, cyclohexyl,
2-ethylhexyl, and n-octyl. R33 is a monovalent hydrocarbon group of
1 to 18 carbon atoms, preferably 1 to 10 carbon atoms, which may contain
a heteroatom such as oxygen, examples of which include straight, branched
or cyclic alkyl groups and substituted forms of such alkyl groups in
which some hydrogen atoms are replaced by hydroxyl, alkoxy, oxo, amino,
alkylamino or the like. Illustrative examples of the substituted alkyl
groups are shown below.

##STR00048##

[0074] A pair of R31 and R32, R31 and R33, or R32
and R33 may bond together to form a ring with the carbon and oxygen
atoms to which they are attached. Each of R31, R32 and R33
is a straight or branched alkylene group of 1 to 18 carbon atoms,
preferably 1 to 10 carbon atoms when they form a ring, while the ring
preferably has 3 to 10 carbon atoms, more preferably 4 to 10 carbon
atoms.

[0076] Also included are substituent groups having the formulae (A-1)-1 to
(A-1)-10.

##STR00049##

[0077] Herein R37 is each independently a straight, branched or
cyclic C1-C10 alkyl group or C6-C20 aryl group,
R38 is hydrogen or a straight, branched or cyclic C1-C10
alkyl group, R39 is each independently a straight, branched or
cyclic C2-C10 alkyl group or C6-C20 aryl group, and
a1 is as defined above.

[0078] Of the acid labile groups of formula (A-2), the straight and
branched ones are exemplified by the following, groups having formulae
(A-2)-1 to (A-2)-69.

[0079] Of the acid labile groups of formula (A-2), the cyclic ones are,
for example, tetrahydrofuran-2-yl, 2-methyltetrahydrofuran-2-yl,
tetrahydropyran-2-yl, and 2-methyltetrahydropyran-2-yl.

[0080] Other examples of acid labile groups include those of the following
formula (A-2a) or (A-2b) while the polymer may be crosslinked within the
molecule or between molecules with these acid labile groups.

##STR00057##

[0081] Herein R40 and R41 each are hydrogen or a straight,
branched or cyclic C1-C8 alkyl group, or R40 and R41,
taken together, may form a ring with the carbon atom to which they are
attached, and R40 and R41 are straight or branched
C1-C8 alkylene groups when they form a ring. R42 is a
straight, branched or cyclic C1-C10 alkylene group. Each of b1
and d1 is 0 or an integer of 1 to 10, preferably 0 or an integer of 1 to
5, and c1 is an integer of 1 to 7. "A" is a (c1+1)-valent aliphatic or
alicyclic saturated hydrocarbon group, aromatic hydrocarbon group or
heterocyclic group having 1 to 50 carbon atoms, which may be separated by
a heteroatom or in which some hydrogen atoms attached to carbon atoms may
be substituted by hydroxyl, carboxyl, carbonyl groups or fluorine atoms.
"B" is --CO--O--, --NHCO--O-- or --NHCONH--.

[0082] Preferably, "A" is selected from divalent to tetravalent, straight,
branched or cyclic C1-C20 alkylene, alkyltriyl and alkyltetrayl
groups, and C6-C30 arylene groups, which may contain a
heteroatom or in which some hydrogen atoms attached to carbon atoms may
be substituted by hydroxyl, carboxyl, acyl groups or halogen atoms. The
subscript c1 is preferably an integer of 1 to 3.

[0083] The crosslinking acetal groups of formulae (A-2a) and (A-2b) are
exemplified by the following formulae (A-2)-70 through (A-2)-77.

##STR00058##

[0084] In formula (A-3), R34, R35 and R36 each are a
monovalent hydrocarbon group, typically a straight, branched or cyclic
alkyl group, which may contain a heteroatom such as oxygen, sulfur,
nitrogen or fluorine. A pair of R34 and R35, R34 and
R36, or R35 and R36 may bond together to form a
C3-C20 aliphatic ring with the carbon atom to which they are
attached.

[0086] Other exemplary tertiary alkyl groups include those of the
following formulae (A-3)-1 to (A-3)-18.

##STR00059## ##STR00060## ##STR00061##

[0087] Herein R43 is each independently a straight, branched or
cyclic C1-C6 alkyl group or C6-C20 aryl group,
typically phenyl, R44 and R46 each are hydrogen or a straight,
branched or cyclic C1-C20 alkyl group, and R45 is a
straight, branched or cyclic C1-C8 alkyl group,
C6-C20 aryl group, typically phenyl, or C2-C20
alkenyl group.

[0088] The polymer may be crosslinked within the molecule or between
molecules with groups having R47 which is a di- or multi-valent
alkylene or arylene group, as shown by the following formulae (A-3)-19
and (A-3)-20.

##STR00062##

[0089] Herein R43 is as defined above, R47 is a straight,
branched or cyclic C1-C20 alkylene group or arylene group,
typically phenylene, which may contain a heteroatom such as oxygen,
sulfur or nitrogen, and e1 is an integer of 1 to 3.

[0090] R30, R33, and R36 in formulae (A-1), (A-2), and
(A-3) may be substituted or unsubstituted aryl groups such as phenyl,
p-methylphenyl, p-ethylphenyl, and alkoxy-substituted phenyl, typically
p-methoxyphenyl, aralkyl groups such as benzyl and phenethyl, or alkyl or
oxoalkyl groups which may contain an oxygen atom, and substituted forms
in which a carbon-bonded hydrogen atom is substituted by hydroxyl or two
hydrogen atoms are replaced by an oxygen atom to form carbonyl. The
substituted alkyl and oxoalkyl groups are as shown below.

##STR00063##

[0091] Of recurring units having acid labile groups of formula (A-3),
recurring units of (meth)acrylate having an exo-form structure
represented by the formula (A-3)-21 are preferred.

##STR00064##

Herein, R.sup.α is hydrogen or methyl; Rc3 is a straight,
branched or cyclic C1-C8 alkyl group or an optionally
substituted C6-C20 aryl group; Rc4 to Rc9, Rc12
and Rc13 are each independently hydrogen or a monovalent
C1-C15 hydrocarbon group which may contain a heteroatom; and
Rc10 and Rc11 are hydrogen. Alternatively, a pair of Rc4
and Rc5, Rc6 and Rc8, Rc6 and Rc9, Rc7 and
Rc9, Rc7 and Rc13, Rc8 and Rc12, Rc10 and
Rc11, or Rc11 and Rc12, taken together, may form a ring,
and in that event, each ring-forming R is a divalent C1-C15
hydrocarbon group which may contain a heteroatom. Also, a pair of
Rc4 and Rc13, Rc10 and Rc13, or Rc6 and Rc8
which are attached to vicinal carbon atoms may bond together directly to
form a double bond. The formula also represents an enantiomer.

[0092] The ester form monomers from which recurring units having an
exo-form structure represented by formula (A-3)-21 are derived are
described in U.S. Pat. No. 6,448,420 (JP-A 2000-327633). Illustrative
non-limiting examples of suitable monomers are given below.

##STR00065## ##STR00066##

[0093] Also included in the acid labile groups of formula (A-3) are acid
labile groups of (meth)acrylate having furandiyl, tetrahydrofurandiyl or
oxanorbornanediyl as represented by the following formula (A-3)-22.

##STR00067##

Herein, R.sup.α is as defined above; Rc14 and Rc15 are
each independently a monovalent, straight, branched or cyclic
C1-C10 hydrocarbon group, or Rc14 and Rc15, taken
together, may form an aliphatic hydrocarbon ring with the carbon atom to
which they are attached. Rc16 is a divalent group selected from
furandiyl, tetrahydrofurandiyl and oxanorbornanediyl. Rc17 is
hydrogen or a monovalent, straight, branched or cyclic C1-C10
hydrocarbon group which may contain a heteroatom.

[0094] Examples of the monomers from which the recurring units substituted
with acid labile groups having furandiyl, tetrahydrofurandiyl and
oxanorbornanediyl are derived are shown below. Note that Me is methyl and
Ac is acetyl.

##STR00068## ##STR00069## ##STR00070## ##STR00071## ##STR00072##

[0095] In the first and second resist compositions used herein, the
polymers may have further copolymerized therein recurring units (i1),
(i2) or (i3) having a sulfonium salt, represented by the general formula
(4). It is noted that these units are collectively referred to as units
(i).

##STR00073##

Herein R20, R24, and R28 each are hydrogen or methyl.
R21 is a phenylene group, --O--R-- or --C(═O)--Y0--R--
wherein Y0 is an oxygen atom or NH, and R is a straight, branched or
cyclic C1-C6 alkylene, phenylene, or alkenylene group, which
may contain a carbonyl (--CO--), ester (--COO--), ether (--O--) or
hydroxyl radical. R22, R23, R25, R26,
R27R29, R30, and R31 are each independently a
straight, branched or cyclic C1-C12alkyl group which may
contain a carbonyl, ester or ether radical, or a C6-C12 aryl
group, C7-C20 aralkyl group or thiophenyl group. Z is a single
bond, methylene, ethylene, phenylene, fluorinated phenylene,
--O--R32--, or --C(═O)--Z1--R32-- wherein Z1 is
an oxygen atom or NH, and R32 is a straight, branched or cyclic
C1-C6 alkylene group, phenylene group or alkenylene group,
which may contain a carbonyl, ester, ether or hydroxy radical. M.sup.- is
a non-nucleophilic counter ion. The subscripts i1, i2 and i3 are numbers
in the range: 0≦i1≦0.3, 0≦i2≦0.3,
0≦i3≦0.3, and 0<i1+i2+i3≦0.3.

[0096] Examples of the non-nucleophilic counter ion represented by Minclude halide ions such as chloride and bromide ions;
fluoroalkylsulfonate ions such as triflate,
1,1,1-trifluoroethanesulfonate, and nonafluorobutanesulfonate;
arylsulfonate ions such as tosylate, benzenesulfonate,
4-fluorobenzenesulfonate, and 1,2,3,4,5-pentafluorobenzenesulfonate;
alkylsulfonate ions such as mesylate and butanesulfonate; imidates such
as bis(trifluoromethylsulfonyl)imide, bis(perfluoroethylsulfonyl)imide,
and bis(perfluorobutylsulfonyl)imide; and methidates such as
tris(trifluoromethylsulfonyl)methide and
tris(perfluoroethylsulfonyl)methide.

[0097] Other non-nucleophilic counter ions include sulfonates having
fluorine substituted at α-position as represented by the general
formula (K-1) and sulfonates having fluorine substituted at α- and
β-positions as represented by the general formula (K-2).

##STR00074##

In formula (K-1), R102 is hydrogen, or a straight, branched or
cyclic C1-C20 alkyl group, C2-C20 alkenyl group, or
C6-C20 aryl group, which may have an ether, ester, carbonyl
radical, lactone ring or fluorine. In formula (K-2), R103 is
hydrogen, or a straight, branched or cyclic C1-C30 alkyl or
acyl group, C2-C20 alkenyl group, or C6-C20 aryl or
aryloxy group, which may have an ether, ester, carbonyl radical or
lactone ring.

[0098] The first base polymer in the first resist composition should be
insoluble in C3-C8 alcohol solvents. To be insoluble in these
solvents, the first polymer should preferably have an adhesive group
selected from among lactone ring, carbonate, thiocarbonate, carbonyl,
cyclic acetal, ether, ester, sulfonic acid ester, cyano, amide groups,
and --O--C(═O)--Y-- wherein Y is sulfur or NH. Since recurring units
(a) facilitate, dissolution in C3-C8 alcohol solvents,
desirably these units should not be introduced into the first polymer, or
if introduced, should be limited to a copolymerization ratio of 20 mol %
or less. In contrast, since recurring units (a) are introduced into the
second polymer to ensure solubility in the C3-C8 alcohol
solvents, it is acceptable to introduce into the second polymer an
adhesive group selected from among lactone ring, carbonate,
thiocarbonate, carbonyl, cyclic acetal, ether, ester, sulfonic acid
ester, cyano, amide groups, and --O--C(═O)--Y--. It is rather
recommended to introduce such an adhesive group into the second polymer
because the adhesion of the second polymer is improved.

[0099] The first base polymer in the first resist composition may have
copolymerized therein recurring units (b2) having an acid labile group
and recurring units (d) having an adhesive group selected from among
lactone ring, carbonate, thiocarbonate, carbonyl, cyclic acetal, ether,
ester, sulfonic acid ester, cyano, amide groups, and --O--C(═O)--Y--
wherein Y is sulfur or NH. The first resist film has the function of an
antireflective coating (BARC) as well. For this function, strong
absorption at the exposure wavelength is necessary. Specifically, the
film should have such absorption as represented by a k value of 0.1 to
1.1, preferably 0.2 to 1.0. In the case of ArF excimer laser having an
exposure wavelength of 193 nm, benzene ring has strong absorption in this
wavelength band. In the first base polymer, benzene ring must be
introduced into 30 to 100 mol % of recurring units. The benzene ring may
be introduced as an acid labile group or as a phenolic hydroxyl group or
adhesive group having lactone.

[0100] The first resist film due to a high aromatic content has high
etching resistance and high barrier properties during ion implantation,
and offers a high function as a mask against both etching and ion
implantation. Although the second resist film has only etching resistance
comparable to conventional ArF resist materials, a combination of the
second resist film with the first resist film provides high resistance.

[0101] In the case of conventional BARC, since the BARC film is processed
by dry etching through the resist pattern as a mask, it must have a high
etching rate. This means that the BARC film has little etch resistance.
In contrast, the first resist film has the same antireflective effect as
BARC, is processed with a developer to form a pattern, so eliminating the
risk of the upper resist film being damaged by dry etching, and has high
etching resistance.

[0104] It is noted that the meaning of a+b+c+d=1, for example, is that in
a polymer comprising recurring units (a), (b), (c) and (d), the sum of
recurring units (a), (b), (c) and (d) is 100 mol % based on the total
amount of entire recurring units. The meaning of a+b+c+d<1 is that the
sum of recurring units (a), (b), (c) and (d) is less than 100 mol % based
on the total amount of entire recurring units, indicating the inclusion
of other recurring units, for example, units (e), (f), (g), (h) and (i).

[0105] The polymer should preferably have a weight average molecular
weight (Mw) in the range of 1,000 to 500,000, and more preferably 2,000
to 30,000, as measured in tetrahydrofuran (THF) or dimethylformamide
(DMF) solvent by GPC using polystyrene standards. Outside the range, a
lower Mw may lead to a lowering in the efficiency of thermal crosslinking
of resist material after development whereas a polymer with too high a Mw
may lose alkali solubility and have a likelihood of footing after pattern
formation.

[0106] If a polymer has a wide molecular weight distribution or dispersity
(Mw/Mn), which indicates the presence of lower and higher molecular
weight polymer fractions, there is a possibility that foreign matter is
left on the pattern or the pattern profile is degraded. The influences of
molecular weight and dispersity become stronger as the pattern rule
becomes finer. Therefore, the multi-component copolymer should preferably
have a narrow dispersity (Mw/Mn) of 1.0 to 2.0, especially 1.0 to 1.5, in
order to provide a resist composition suitable for micropatterning to a
small feature size.

[0107] It is understood that a blend of two or more polymers which differ
in compositional ratio, molecular weight or dispersity is acceptable.

[0108] The polymers as used herein may be synthesized by any desired
method, for example, by dissolving unsaturated bond-containing monomers
corresponding to the respective units (a), (b), (c), (d), (e), (f), (g),
(h), and (i) in an organic solvent, adding a radical initiator thereto,
and effecting heat polymerization. Examples of the organic solvent which
can be used for polymerization include toluene, benzene, tetrahydrofuran,
diethyl ether and dioxane. Examples of the polymerization initiator used
herein include 2,2'-azobisisobutyronitrile (AIBN),
2,2'-azobis(2,4-dimethyl-valeronitrile), dimethyl
2,2-azobis(2-methylpropionate), benzoyl peroxide, and lauroyl peroxide.
Preferably the system is heated at 50 to 80° C. for polymerization
to take place. The reaction time is 2 to 100 hours, preferably 5 to 20
hours. The acid labile group that has been incorporated in the monomer
may be kept as such, or the acid labile group may be once eliminated with
an acid catalyst and thereafter protected or partially protected. The
polymer used as the base resin is not limited to one type and a mixture
of two or more polymers may be added. The performance of a resist
material may be adjusted by using plural polymers.

[0109] The first and second positive resist compositions may further
comprise an organic solvent, an acid generator, and optionally, a
dissolution regulator, basic compound, surfactant, acetylene alcohol, and
other components.

[0110] Each of the first and second positive resist compositions used
herein may include an acid generator in order for the composition to
function as a chemically amplified positive resist composition. Typical
of the acid generator used herein is a photoacid generator (PAG) capable
of generating an acid in response to actinic light or radiation. The PAG
is any compound capable of generating an acid upon exposure to
high-energy radiation. Suitable PAGs include sulfonium salts, iodonium
salts, sulfonyldiazomethane, N-sulfonyloxyimide, and oxime-O-sulfonate
acid generators. The PAGs may be used alone or in admixture of two or
more.

[0115] Examples of the basic compound which can be used in the first and
second resist compositions are described in JP-A 2008-111103, paragraphs
[0146] to [0164], and exemplary surfactants in paragraphs [0165] to
[0166]. Exemplary dissolution regulators are described in JP-A
2008-122932 (US 2008090172), paragraphs [0155] to [0178], and exemplary
acetylene alcohols in paragraphs [0179] to [0182].

[0116] Notably, an appropriate amount of the acid generator is 0.5 to 30
parts, preferably 1 to 20 parts by weight, an appropriate amount of the
organic solvent is 100 to 10,000 parts, preferably 300 to 8,000 parts by
weight, and an appropriate amount of the basic compound is 0.0001 to 30
parts, preferably 0.001 to 20 parts by weight, per 100 parts by weight of
the base polymer.

[0117] Now referring to the drawings, the pattern forming process of the
invention is illustrated in FIG. 1. A first positive resist composition
is coated onto a processable substrate 20 disposed on a substrate 10 to
form a first resist film 31 as shown in FIG. 1A. A second positive resist
composition is coated on first resist film 31 to form a second resist
film 32 as shown in FIG. 1B. After every application of the first and
second resist compositions, the coating is baked for drying, i.e.,
removal of the solvent for thereby preventing intermixing between the
first and second resist films and preventing the first resist film from
being dissolved when the second resist composition is applied thereon.
The bake temperature is preferably in a range of 60 to 180° C.,
more preferably 70 to 150° C. The first resist film typically has
a thickness in the range of 5 to 100 nm while a thickness sufficient to
achieve a satisfactory antireflective effect is preferably selected. This
is followed by exposure, development and etching steps as shown in FIGS.
1C, 1D, and 1E, respectively.

[0118] In the case of the prior art pattern forming process using DBARC,
the first resist film in FIG. 1 corresponds to the DBARC. When DBARC is
used, a DBARC solution is coated and baked at a high temperature of about
200° C. to induce crosslinking reaction for thereby preventing
dissolution and intermixing when a resist solution is coated thereon.
However, the crosslinking reaction results in a substantial loss of
solubility in developer, leaving a problem of scumming in the space
portion.

[0119] In the pattern forming process using BARC, since BARC is not
photosensitive, the exposed portion of BARC is not dissolved even after
development of the resist film. In order to open the processable
substrate surface in the exposed portion, the BARC is opened by dry
etching through the resist pattern after development as a mask. Thus the
thickness of the resist film has been reduced at the stage prior to
processing of the processable substrate (or target film).

[0120] FIG. 2 shows another embodiment of the pattern forming process of
the invention. The process of FIG. 2 is the same as that of FIG. 1 until
the development step except that the processable layer is not formed. The
development is followed by ion implantation as shown in FIG. 2E whereby
ions are implanted into the substrate surface in an opening. The pattern
forming process using BARC has the problem that when BARC is opened by
dry etching, the substrate surface is oxidized. Upon ion implantation,
ions are stopped by the oxide film. In the patterning process involving
ion implantation, the substrate surface must be open after development.

[0122] In the embodiment of the invention, the first resist film of the
first positive resist composition is formed on the processable layer
directly or via a silicon-containing intermediate film, and the second
resist film of the second positive resist composition is formed on the
first resist film. The first resist film typically has a thickness in the
range of 5 to 100 nm, preferably 10 to 100 nm, and more preferably 20 to
50 nm while it is desirable to select a film thickness suitable to
minimize substrate reflection. The second resist film typically has a
thickness in the range of 20 to 300 nm, preferably 30 to 250 nm. The
first and second resist films each are prebaked after spin coating and
before exposure. Preferred prebake conditions include a temperature of 60
to 180° C., especially 70 to 50° C. and a time of 10 to 300
seconds, especially 15 to 200 seconds.

[0123] Next exposure is carried out. For the exposure, preference is given
to high-energy radiation having a wavelength of 140 to 250 nm, and
especially ArF excimer laser radiation of 193 nm. The exposure may be
done either in a dry atmosphere such as air or nitrogen stream or by
immersion lithography in water. The ArF immersion lithography uses
deionized water or liquids having a refractive index of at least 1 and
highly transparent to the exposure wavelength such as alkanes as the
immersion solvent. The immersion lithography involves exposing a prebaked
resist film to light through a projection lens, with water introduced
between the resist film and the projection lens. Since this allows lenses
to be designed to a NA of 1.0 or higher, formation of finer feature size
patterns is possible. The immersion lithography is important for the ArF
lithography to survive to the 45-nm node. In the case of immersion
lithography, deionized water rinsing (or post-soaking) may be carried out
after exposure for removing water droplets left on the resist film, or a
protective film may be applied onto the resist film after pre-baking for
preventing any leach-out from the resist film and improving water slip on
the film surface. The resist protective film used in the immersion
lithography is preferably formed from a solution of a polymer having
1,1,1,3,3,3-hexafluoro-2-propanol residues which is insoluble in water,
but soluble in an alkaline developer, in a solvent selected from alcohols
of at least 4 carbon atoms, ethers of 8 to 12 carbon atoms, and mixtures
thereof. After formation of the resist film, deionized water rinsing (or
post-soaking) may be carried out for extracting the acid generator and
the like from the film surface or washing away particles, or after
exposure, rinsing (or post-soaking) may be carried out for removing water
droplets left on the resist film.

[0124] To the first and second resist compositions, an additive for
enhancing water repellency of resist surface may be added. This additive
is a polymer having a fluoroalcohol group, which segregates on the resist
surface after spin coating to reduce the surface energy, improving water
slip. Such polymers are described in JP-A 2007-297590 and JP-A
2008-122932.

[0125] Exposure is preferably performed in an exposure dose of about 1 to
200 mJ/cm2, more preferably about 10 to 100 mJ/cm2. This is
followed by baking (PEB) on a hot plate at 60 to 150° C. for 1 to
5 minutes, preferably at 80 to 120° C. for 1 to 3 minutes.

[0126] Development is then carried out by a standard technique such as
puddle, dip or spray technique using an aqueous alkaline solution,
typically a 0.1 to 5 wt %, preferably 2 to 3 wt % aqueous solution of
tetramethylammonium hydroxide (TMAH). In this way the desired pattern is
formed on the substrate.

[0127] FIGS. 3 to 10 show the results of experiments. For each of first
resist films having different n values varying from 1.3 to 2.0 by an
increment of 0.1, reflection from beneath the resist film is computed
when the k value and film thickness are varied. Exposure is ArF dry
lithography with wavelength 193 nm using NA 0.85 lens and 2/3 annular
illumination.

[0128] The first resist film has optimum values of extinction coefficient
(k value) at the exposure wavelength and film thickness which differ
depending on a particular n value. In the thinnest film region where
reflectivity is 1% or below (depicted as white squares with reflectivity
of 0-0.01), the film is thinner as the n value is greater, and at this
point, the k value is greater.

[0129] Since the first resist film contains a large amount of light
absorber for anti-reflection purpose, a thicker resist film results in a
pattern of tapered profile. It is thus necessary to make the first resist
film as thin as possible. In FIG. 6, for example, although regions giving
a reflectivity of 1% or less exist in film thickness ranges of 30-50 nm
and 90-120 nm, a film thickness range of 30-50 nm is preferably used. In
the range of 30-50 nm, a choice of as thin a film as possible is
preferred. The first resist having a higher refractive index n may
provide a reflectivity of 1% or less in a thinner film form.

EXAMPLE

[0130] Synthesis Examples and Examples are given below together with
Comparative Examples for further illustrating the invention although the
invention is not limited thereby. The weight average molecular weight
(Mw) of a polymer is determined by GPC versus polystyrene standards using
tetrahydrofuran (dimethylformamide for Polymer 1-9) as a solvent.
Dispersity (Mw/Mn) is computed therefrom.

Synthesis Example

[0131] Polymers to be added to resist compositions were synthesized by
combining monomers, effecting copolymerization reaction in
tetrahydrofuran solvent, pouring the reaction mixture into methanol for
crystallization, repeatedly washing with hexane, isolation, and drying.
The resulting Polymers 1-1 to 1-19, Polymers 2-1 to 2-8, and Comparative
Polymer 1-1 are identified below. The composition of a polymer was
analyzed by 1H-NMR spectroscopy, and Mw and Mw/Mn by GPC.

[0132] A first resist composition in solution form was prepared by mixing
a polymer (Polymers 1-1 to 1-19, Polymer 2-1, or Comparative Polymer
1-1), acid generator, and amine quencher in a solvent in accordance with
the formulation shown in Table 1, and filtering through a Teflon®
filter having a pore size of 0.2 μm. The solvent contained 50 ppm of a
surfactant FC-4430 (3M-Sumitomo Co., Ltd.).

[0133] In preparing Comparative Resist 1-3, Polymer 1-1 could not be
dissolved in 2-methyl-1-butanol.

[0137] A second resist composition in solution form was prepared by mixing
a polymer (Polymers 2-1 to 2-8 or Comparative Polymer 1-1), acid
generator, and amine quencher in a solvent in accordance with the
formulation shown in Table 2, and filtering through a Teflon® filter
having a pore size of 0.2 μm. The solvent contained 50 ppm of a
surfactant FC-4430 (3M-Sumitomo Co., Ltd.).

[0138] In preparing Comparative Resist 2-3, Comparative Polymer 1-1 could
not be dissolved in 2-methyl-1-butanol.

[0139] Each of the first resist compositions in Table 1 was coated on a
8-inch silicon wafer and baked at 110° C. for 60 seconds to form a
first resist film of 35 nm thick. Using a variable angle spectroscopic
ellipsometer (VASE®) of J. A. Woollam Co., the refractive index (n)
and extinction coefficient (k) at wavelength 193 nm of the resist film
were determined. The results are shown in Table 3.

[0140] Each of the first resist compositions in Table 1 was coated on a
8-inch silicon wafer and baked at 110° C. for 60 seconds to form a
first resist film of 35 nm thick. A solvent (shown in Table 4) was
statically dispensed on the resist film for 20 seconds, followed by spin
drying at 1,500 rpm and baking at 100° C. for 60 seconds. The
thickness of the resist film was measured for determining a film
thickness reduction (film loss) before and after solvent dispensing. The
results are shown in Table 4.

[0141] The first resist composition in Table 1 was coated on a silicon
substrate (which had been vapor primed with hexamethyl disilazane HMDS)
and baked on a hot plate at 110° C. for 60 seconds to form a first
resist layer of 35 nm thick. The second resist composition in Table 2 was
coated on the first resist film and baked on a hot plate at 100°
C. for 60 seconds to form a second resist layer of 165 nm thick. Using an
ArF excimer laser scanner NSR-S307E (Nikon Corp., NA 0.85, σ 0.93,
2/3 annular illumination, 6% halftone phase shift mask), the resist film
was exposed to a 90 nm line, 180 nm pitch pattern. Immediately after
exposure, the resist film was baked at 100° C. for 60 seconds and
developed with a 2.38 wt % aqueous solution of tetramethylammonium
hydroxide for 30 seconds, obtaining a 90 nm line-and-space pattern. A
cross-sectional profile of the pattern was observed under SEM. The
results are shown in Table 5.

[0142] In Examples 1-1 to 1-26, the pattern of the first and second layer
resist film was formed after development. By virtue of the antireflective
effect of the first resist layer, sidewalls of the second resist layer
pattern were found to be devoid of roughness due to standing waves.

[0143] When a first resist film formed of Comparative Resist composition
1-1 free of absorber component was used, sidewalls of the second resist
layer pattern after development were found to have ridges due to standing
waves by substrate reflection.

[0144] In the example wherein an alcohol solvent-soluble polymer was used
in the first layer resist film, the first layer resist film was dissolved
when the second layer resist composition was coated thereon. Pattern
formation was impossible because of intermixing of the first and second
layer resist films.

[0145] In the example wherein PGMEA solvent capable of dissolving the
first layer resist film was used in the second layer resist composition,
pattern formation was impossible because of intermixing of the first and
second layer resist films.

[0146] While the invention has been described with reference to a
preferred embodiment, it will be understood by those skilled in the art
that various changes may be made and equivalents may be substituted for
elements thereof without departing from the scope of the invention. In
addition, many modifications may be made to adapt a particular situation
or material to the teachings of the invention without departing from the
essential scope thereof. Therefore, it is intended that the invention not
be limited to the particular embodiment disclosed as the best mode
contemplated for carrying out this invention, but that the invention will
include all embodiments falling within the scope of the appended claims.

[0148] Although some preferred embodiments have been described, many
modifications and variations may be made thereto in light of the above
teachings. It is therefore to be understood that the invention may be
practiced otherwise than as specifically described without departing from
the scope of the appended claims.